On board ND's - how do they work
 

Traditionally an ND filter on the front of the lens reduces the amount of light hitting the plate/sensor.

On videocams, how do the on board ND's work? Do they reduce the amount of light, or do they attenuate the gain on the sensor itself?

Specifically interested in the GY-HD100 but I imagine most cams use the same scheme.

They are physical ND filters that are moved in front of the CCD in the case of the FX1/Z1U. Gain control should be a separate function.

I would assume this was the case with the HD100.

-Steve

The DVX also has a physical filter wheel. I'd assume the JVC does as well.